Amplifying electrical variations



July 3, 1928. I 1,675,874

H. C. BAUMANN AMPLIFYING ELECTRICAL VARIATIONS Filed June 4, 1926hue/2f: l/aro/d 6. 5000/0/10 Patented July 3, 1928 UNITED STATES1,675,874 PATENT OFFICE.

HAROLD C. BAU'M A NN, OF WEST LONG-BRANCH, NEW JERSEY, ASSIGNOR TO BELLTEL- E-PHONE LABORA'IORIES, INCORPORATED, OF NEW YORK, N". Y.,- ACORPORATION OF NEW YORK.

Application filed .Tune 4,

This invention relates to amplification of electric waves.

An object of the invention is tov produce high amplification of highfrequencyelecb trical variations without causing disturbance in theamplifying means.

It has been suggested that it is possible to stabilize two highfrequency vacuum tube amplifying valves, against singing due I totunedgrid and anode circuits, by reducing the efficiency of the couplingbetween the valves,v and that a form of amplifier most suitable for Wavelengths over one thousand meters, for which resistance coupling is su'fficiently efficient, is a series of tubes with alternate single tunedcouplings and resistan'ce couplings.

Where, for example, voltage variations of the order of a million cyclesper second are to be highly amplified, a multi-stage vacuum tubeamplifier having transformer coupling between stages is desirable, buthas a marked tendency to sing.

In accordance, however, with the present invention there is provided anamplifier employing a. number of transformer coupled vacuum tubeamplifier stages in tandem, and in order to prevent singing in theamplifier, these stages are arranged in tandem connected groups and inthe connection between each two groups there is included a vacuum tubestage having a resistance coupling to one of the transformer coupledstages. Each resistance coupling reduces the effect of feed back betweenthe stages which it connects, and makes it possible to continue addingstages until the feed back in these additional stages becomes serious,whereupon another resistance coupling is introduced, and so on. Aneleven stage amplifier so constructed has been found to operatesatisfactorily at awavelength of three hundred meters, with a voltageamplification of seventy thousand times,

Other objects and features of the invention will be apparent fromthefollowing description and claims.

As an example of one specific embodiment of the invention, the singlefigure of the drawing shows diagrammatically a double detection radioreceiving system wherein the principles of this invention are applied toan intermediate frequency amplifier,

In the system shown in the drawing, an

AMPLIFYING ELECTRICAL VARIATIONS.

1926. Serial No. 113,607.

antenna- 12, for reeeiving,'for example, radio telephone or telegraphsignals transmitted to the antenna as a carrier wave modulated by signalwaves of audible frequency, is connected to the input circuit of firstdetector 15 indicated by block D, the input circuit of the detectorconstituting a tuning circuit for the antenna. A beating or heterodyneoscillator O is so connected to the detector 15 that the modulatedcarrier wave, received from the antenna and amplified by the firstdetector, modulates or is modulated by waves from the beatingoscillator. One of the modulation products, the so-called intermediatefrequency, is a signal modulated wave of a frequency equal to thedifference between the carrier frequency received by the antenna and theoscillator frequency. This difference will ordinarily be small comparedto the frequency received by the antenna. This intermediate frequency,in the output circuit of detector 15, may be selected by an intermediatefrequency filter, incorporated as a part of the detector circuit, andpassed'to the input circuit of an amplifier A, of the type referred toabove, whereas other frequencies may be suppressed by the filter andhence will not be received by the amplifier. The resulting amplifiedwaves are transmitted from the output circuit of amplifier A to theinput circuit of a second detector 16, indicated by block D, whichdetectsthe amplified wave and derives the audible frequency signal wavestherefrom, so that they produce audible sounds in a telephone 17connected to the output circuit of detector 16. The detector 16, may be,for example, a three electrode vacuum tube detector. I

r The oscillator O and the first detector 15 including the intermediatefrequency filter are preferably of such type, and are preferable sointerconnected that they are adapted for the efiicient reception ofradio signaling waves having frequencies extending up to very high radiofrequencies, of the order of forty megacyeles and may be of any suitabletype. A preferred type is that disclosed in the application of Harald T.Friis, Serial No. 104,619, filed April 26, 1926. Vith the antenna andfirst detector input circuit tuned to a frequency having a value oftwenty million cycles, for example, and the oscillator O tuned to afrequency 7'' equal to fi1,000,000 cycles per second, for example, theintermediate frequency amplifier may have a resonant frequency of onemillion cycles per second, and a band width of, say, 30 to, 50kilocycles, which is a width sufficient to avoid any necessity for hairline tuning at high signaling frequencies.

' One advantage of employing such a high intermediate frequency in shortwave recoupling is a resistance coupling and the others are transformercouplings. The output circuit of tube 11 is coupled to the input circuitof detector 16 by a coupling transformer 31. The input circuit ofdetector 16 includes a shunt condenser 32. Each of the couplingtransformers has a primary Wind'- ing and a secondary winding, of #38German silver resistance wire, and tunes with the effective reactance ofthe input circuit of the succeeding tube at a wave length of about 300meters. The primary and secondary are wound as a bifilar coil on aWooden spool, the transformers thus having-a 1 to 1 ratio. The spoolhasa diameter of 2 and a width of the winding slot being deep and 4;wide. The primary and due to the filament heating current.

secondary windings. are turns each. The transformers are placed incopper cans 35, each 2 X 2 X 1%, in order to reduce the effectof themagnetic field,

Each of theresistance couplings comprises a resistance 36 in the platecircuit of the preceding tube, a high capacity direct current. blockingcondenser 38 connected between the plate of that tube and the grid ofthe succeeding tube to prevent application of steady potential from thespace current source '43 to that grid, and a high resistance 37 forminga direct current conductive grid leak path from the grid to the filamentcircuit. The filaments of tubes 1 to 11 are all connected to a source ofelectromotive force 40, in series with each other and with aresistance41 which is in circuit between the filament of tube 1 and thenegative pole of source 40. The grid of tube 1 may be conductivelyconnected to the negative pole of source 40 to obtain a steady biasingpotential.

from the voltage drop across resistance 41 Each of the tubes 1 to 11 hasits grid connected through its grid leak resistance or through thesecondary winding of its input transformer, to the low potential end ofthe fila In the particular amplifier described in i detail herein, eachof the tubes 1 to 11' may be a Western Electric Company type 215A vacuumtube. A 45 volt source of electromotive force 42 supplies space currentfor all of the tubes having their plates connected to outputtransformers. A 67 volt battery 43 supplies space current for theremaining tubes, the voltage supplied to the tubes from this sourcebeing adjustable in value, as indicated in the drawing by the movablecontact 44 adjustable to different cells of the battery, in order tocontrol the gain of the amplifier. It was found that by thus varying theplate voltage of the tubes having their output circuits resistancecoupled to other tubes, the amplifier gain was varied without changingthe shape or band width i of the transmission curve or resonance curveof the amplifier plotted between the logarithm of the voltageamplification, as ordinates, and frequently as abscissa By band Width ismeant the width of the resonance curve at half of its maximum height.

Condensers 45 and 46 by-pass high frequency around sources 42 and 43respectively and reduce any tendency to" sing which might result fromvoltage drop across the common battery due to passage of the highfrequency current through the battery. V

Condensers 47 and 48 connect points of the filament circuit to ground,for 'high' frequency, in order to reduce any tendencyto sing due to flowof high frequency current in the filament circuit. The resistancecouplings in the circuit also diminish such tendency.

amplification obtained was 4.5 times, and the band width 700,000 cyclesper second. Three stages gave an amplification of over 100 times andwith fourstages the amplifier went into the singing condition. With twostages of amplification, at 10 cycles per second, a small amount offeedback was present, and with three stages considerably more. However,two stages were quite stable. Experiments showed that a single stageresistance coupled amplifier gave an amplification of about unity at onemillion cycles per Each of the condensers 45 to 48 may have a capacityof the order of one microfaradg With a single stage of amplificationusing the transformer described above, the voltage second. Connectingone pair of transformer coupled amplifying stages to another pair oftransformer coupled amplifying stages by a resistance coupled amplifierstage eliminated singing and gave an amplification of 360 times, whichis merely the proper amplification for four stages assuming that theamplification of the transformer coupled stages was 4.5 times per stageand the amplification of the resistance coupled stage unity.

Inserting a resistance coupled amplifier between each pair of a numberof pairs of transformer coupled amplifying stages, in this manner, aneleven stage amplifier was built, which gave a voltage amplification of70,000 times with a band width of 50,000 cycles per second.

Specific values have been mentioned above for the purpose ofillustration, and the invention is not to be regarded as thereby limitedto those values or limited in any other respect except as indicated bythe scope of the appended claims.

V hat is claimed is:

1. an amplifier adapted to produce, without singing, high gain at afrequency of the order of at least one million cycles per second havinga plurality of amplifier stages in tandem, each of said stages includingan electric space discharge device having input and output circuits,said amplifier comprising resistance coupling means coupling two of saiddevices in tandem, and said amplifier comprising means coupling theinput circuit of one of said two devices to another of the d vices, saidlast means and circuit having a natural period corresponding to afrequency of the order of at least one million cycles per second, andsaid amplifier also comprising means coupling the output path of theother of said two devices to the input circuit of another of thedevices, said last mentioned means and circuit having a natural periodcorresponding to a frequency of the order of at least one million cyclesper second.

2. An amplifier circuit for producing without singing a high gain atfrequencies of the order of at least one million cycles per scondcomprising a plurality of electric discharge amplifiers each havinginput and output circuits, interstage transformers for coupling saidamplifiers in tandem into groups each comprising at least twoamplifiers, and resistance means connected in shunt to the Outputcircuit of the last amplifier in one of said groups and in shunt to theinput circuit of the first amplifier of another of said groups forcoupling said groups in tandem.

3. An amplifier circuit for producing without singing a high gain atfrequencies of the order of at least one million cycles per secondcomprising a plurality of electric discharge amplifiers each havinginput and output circuits, interstage transformers for coupling saidamplifiers in tandem into groups each comprising at least twoamplifiers, resistance means connected in shunt to the output circuit ofthe last amplifier in one of said groups and in shunt to the inputcircuit of the first amplifier of another of said groups for couplingsaid groups in tandem, and a variable potential source of currentconnected to supply space current to said last amplifier of said onegroup to control the overall gain of the said amplifier circuit.

In witness whereof, I hereunto subscribe my name this 2nd day of June A.D., 1926.

HAROLD C. BAUMANN.

